Conventionally, in an internal combustion engine, spark plugs are used to ignite fuel mixtures. As is shown in FIG. 21, a general spark plug includes a center electrode, an insulating member which holds the center electrode in an axial hole, a metal shell which surrounds the periphery of the insulating member to hold the insulating member, and a ground electrode which is joined to the metal shell at a proximal end portion and which forms a spark gap at a distal end portion with the center electrode. The fuel mixture is ignited by a spark discharge occurring in the spark gap. Although the form of the spark plug shown in FIG. 21 is of a so-called projecting type, in addition to this, there are spark plugs of a slant type and a semi-creeping type (refer to JP-A-6-176849).
In recent years, valve diameters of intake valves and exhaust valves are required to be extended to increase the output of an internal combustion engine. A larger water jacket is also required to be equipped on such an internal combustion engine whose output is increased in that way in order to cool the engine with good efficiency. However, with these required countermeasures implemented, since a space where to install spark plugs, which are to be install in the internal combustion engine, becomes small, spark plugs with smaller diameters are now required.
However, in the event that the diameter of a spark plug is reduced simply, an insulating distance between an insulating member and a metal shell is narrowed. Because of this, depending upon how carbon deposits are accumulated on the insulating member, a lateral spark in which a spark occurs from the center electrode to the metal shell via an insulator or an inside spark in which a spark occurs from the center electrode to the metal shell through a gap between the insulator and the metal shell is generated (refer to FIG. 21). When lateral sparks and inside sparks occur frequently, the frequency of discharge at the normal spark gap decreases, thus, there is a problem of ignition with fuel mixture.
In relation to these problems, for example, JP-A-2006-49207 discloses a spark plug for suppression of the lateral spark in which an outside diameter of a front end of an insulating member is formed so as to be increased gradually from a front end side to a rear end side and a volume from the front end of the insulating member to a position lying 0.1 mm rearwards from the front end is 0.38 mm3 or lower. JP-A-2000-243535 discloses a spark plug including a center electrode having a high melting point metal tip, wherein a thickness of a portion of an insulating member which is positioned to face a front end face of a metal shell is 1.1 mm or larger and further, an outside diameter of a portion of the center electrode which is positioned to face a front end of the insulating member is 1.4 mm or larger and 2.0 mm or smaller.